import java.util.Scanner;

public class A {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		if(a == b) {
			System.out.print(-1);
			return;
		}
		System.out.print(6 - (a + b));

	}
}
